KÁRMÁN LINE - Border of Earth and Space
Above Earth's surface, our atmosphere is divided into five layers, the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Chances are, unless you've spent time as a fighter pilot, you've never gone beyond the troposphere. And all humans except for the 24 astronauts who have visited the moon have ever ventured beyond the thermosphere.
When you reach 50 miles of altitude, near the border between the mesosphere and the thermosphere, that's where aerodynamic control surfaces stop working (you'll need rockets to steer). And for record-keeping and giving out astronaut wings, the Kármán Line, located around 62 miles (100 km) above the surface of the Earth, serves as a rough space border: this is where a craft begins to escape the grip of our planet's gravity.
As you fly higher into the atmosphere, the air gets thinner, and this means a plane needs more speed for its wings to generate the lift needed to keep it aloft. The Kármán Line is the point where the speed needed to maintain altitude is equal to escape velocity: the speed at which a craft ceases to follow the curvature of the Earth, and the craft begins to enter space.